Engine systems may be configured with exhaust gas recirculation (EGR) systems via which at least a portion of the exhaust gas is recirculated to the engine intake. Various sensors may be coupled in the engine system to estimate the amount of EGR being delivered to the engine. These may include, for example, various temperature, pressure, oxygen, and humidity sensors. Since the accuracy of the EGR estimation relies on the correct functioning of the various sensors, periodic sensor diagnostics are used.
One example approach for diagnosing a humidity sensor is illustrated by Xiao et al. in U.S. Pat. No. 7,715,976. Therein, humidity sensor degradation is determined based on a comparison of an intake humidity estimated by a first humidity sensor in the intake manifold with an exhaust humidity estimated by a second humidity sensor in the exhaust manifold and an ambient humidity estimated by a third humidity sensor located outside of the engine. The sensor readings are compared during conditions when all the sensor readings are expected to be substantially equal, such as during engine non-fueling conditions in which the EGR valve is closed. If the readings of the three humidity sensors differ by more than a threshold, humidity sensor degradation may be determined. For example, if the ambient humidity and the exhaust humidity are substantially equal, and the intake humidity differs by greater than a threshold amount from them, degradation of the intake humidity sensor may be determined.
However, the inventors herein have identified a potential issue with such an approach. The accuracy of determining degradation of any one humidity sensor may depend on the proper functioning of the other humidity sensors. Further, multiple humidity sensors may not be needed for engine control. For example, the inventors herein have recognized that even in a dual intake path system, it may be possible to effectively operate the engine with reduced emissions using asymmetric humidity sensing.
Thus in one example, the above issue may be at least partly addressed by a method of operating an engine including a humidity sensor positioned downstream of an EGR throttle valve. In one embodiment, the method comprises, closing the EGR throttle valve, and indicating humidity sensor degradation based on each of a change in intake air relative humidity and pressure responsive to the EGR throttle valve closing.
For example, during selected engine operating conditions, an EGR throttle valve may be temporarily closed while torque disturbances are compensated for by corresponding adjustments to a downstream air intake throttle valve. Since ambient air entering the engine intake is at atmospheric pressure, the air pressure downstream of the EGR throttle valve decreases as the EGR throttle valve closes. Thus, by changing the position of the EGR throttle valve, air pressure experienced at a humidity sensor, positioned downstream of the EGR throttle valve, may be changed. Since relative humidity is a measure of the percentage of water vapor per area at a specific pressure, the relative humidity is expected to change in accordance with the change in air pressure. An engine controller may compare a change in the relative humidity, as estimated by the humidity sensor, with the change in the intake air pressure, as estimated by a pressure sensor positioned in the intake manifold, downstream of the EGR throttle valve. If the change in relative humidity is not proportional to the change in air pressure (as determined by a difference or ratio of the change in humidity to the change in pressure), sensor degradation may be determined and a corresponding diagnostic code may be set. In other words, by utilizing the pressure effect generated on the humidity sensor, it is possible to correlate proper humidity sensor operation with the change in pressure. Further, by using the EGR throttle to generate the pressure change, it is possible to still maintain engine torque by a corresponding adjustment to the air intake throttle.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.